After one of the most difficult operations attempted on conjoined twins in Britain, surgeons have succeeded in saving only one of them.

Great Ormond Street Hospital for Children, where the operation took place, said surgeons had worked for 11 hours on Monday to separate the Williams sisters, Hope and Faith but Hope's lungs had proved too small to support her breathing.

The operation began on Monday morning and was complete by 7pm but details of the outcome were not released until yesterday. Hope died on Monday evening, before the surgery was complete but the announcement was delayed because doctors feared Faith might not survive the night.

Hope's death was not unexpected; as the smaller of the two, her lungs were weak. The surgeons had wanted to delay the operation to give her a chance to gain strength but a blockage in their joined intestine caused a "sudden deterioration" in the twins' health.

Professor Agostino Pierro, who led the team of surgeons, said it was "one of the most complex and challenging" operations the team had faced.

The twins were joined from the breast bone to the navel and shared a liver. They had separate hearts and lungs and four limbs each, but their blood vessels, and hence their circulatory systems, were linked.

Separating the blood vessels and joining them up again so that each twin had her own circulatory system was the biggest challenge and accounted for the length and complexity of the operation.

The twins' parents, Laura Williams, 18, and her husband Aled, 28, from Shrewsbury, Shropshire, were present at the hospital for the operation.

They were taken to see Hope after she had been separated from her sister. She was on a ventilator and died in their presence.

"This was an emergency operation because there was a blockage in their joined intestine which could only be resolved through separation," Professor Pierro said.

"The technical surgery worked well, although it was extremely challenging, and went according to plan.

"However, very sadly, after separation, baby Hope's lungs proved too small to support her breathing and she died last night in the presence of her parents. They are clearly devastated by the loss of their daughter and we offer them our deepest condolences on their loss.

"Baby Faith is stable after separation. She requires support for her breathing but she is gradually improving. However, it is early days and complications can occur."

The next days will prove crucial for Faith, whose abdomen was left open after the separation. Her condition is likely to fluctuate hourly, as with all babies treated in intensive care.

She will be monitored around the clock by teams of doctors and nurses who will adjust her drugs and fluids according to how she responds. Professor Pierro said: "If everything goes well, there will be more surgery required to close her tummy but I can't tell you now when the surgery will happen. It's fair to say the operation done on Hope and Faith was one of the most complex and challenging we have ever faced."

A risky business: The dangers of surgically separating twins

*Seperating conjoined twins is always a risky procedure, depending on how many organs are shared. Some cases are inoperable.

Experience at Great Ormond Street shows that where the separation is performed as an emergency, one in four babies survives.

Where it is planned, the survival rate rises to 80 per cent. The operation on the Williams sisters was a planned procedure brought forward 24 hours.

Each twin had a heart defect, their circulatory systems were intertwined and they were so premature that their single liver was not easy to divide.

Although the operation was a technical success, as often in these cases it was impossible to tell how the twins would fare after separation.

The surgeon, Edward Kiely, who operated on the twins with Professor Angelino Pierro, is the most experienced in the country, having treated 16 previous cases.

A spokesman for Great Ormond Street said the team hoped to save both twins but Hope had started out the weaker of the two. "Having separated them, you never know how the circulation will work. They were saddened but not surprised by Hope's death," he said.